Why does the Pacific Ocean have a "Ring of Fire"?

Because the Pacific is almost entirely ringed by subduction zones. In each one, an ocean plate dives under its neighbour and sinks back into the mantle. That ring is about 40,000 kilometres long. It holds around two-thirds of the world's volcanoes. About 90 percent of all earthquakes happen there too. It isn't one structure, but many plates sinking all around the same ocean.

Why do plates sink at all? Cold ocean plate is slightly denser than the hot, soft rock beneath it. Once it starts going down, it keeps sinking under its own weight. It can move up to 11 centimetres a year. As far as we know, Earth is the only planet where this happens.

The sinking plate carries water locked in its minerals. Around 100 kilometres down, that water escapes into the mantle above. It lowers the melting point, and magma forms. The magma rises to feed chains of volcanoes called volcanic arcs, like the one that includes Mount Fuji. Because this magma is so rich in water, arc volcanoes tend to be extremely explosive.

Subduction also makes the deepest and biggest earthquakes. Most earthquakes start less than 20 kilometres down. In subduction zones, they can start as deep as 600 kilometres. Nine of the ten largest earthquakes of the past century struck subduction zones. They include Chile in 1960, the largest ever recorded. In 1964, geologist George Plafker studied Alaska's huge Good Friday earthquake. He showed that Pacific crust was being forced down beneath Alaska. That finding helped build the theory of plate tectonics.